Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used. In particular, techniques for developing a high capacity and high power negative electrode active material have been required as the lithium secondary battery market has recently expanded from small-sized lithium secondary batteries used in portable devices to large-sized secondary batteries used in vehicles. Thus, development of non-carbon-based negative electrode active materials based on materials, such as silicon, tin, germanium, zinc, and lead, having a higher theoretical capacity than a carbon-based negative electrode active material has been conducted.
Among the above materials, since a silicon-based negative electrode active material has a capacity (4,190 mAh/g) 11 times or more higher than a theoretical capacity (372 mAh/g) of a carbon-based negative electrode active material, the silicon-based negative electrode active material is on the spotlight as a material for replacing the carbon-based negative electrode active material. However, since the volume expansion of the silicon-based negative electrode active material during the intercalation of lithium ions is 3 times or more when silicon is only used, the collapse of the silicon-based negative electrode active material occurs as charging and discharging of the battery proceed. As a result, the capacity may be reduced by losing electrical contacts.
Thus, in order to address the above limitation, a method of using nano-sized silicon, a method of using rod or fiber-shaped silicon, or a method of using porous silicon has been proposed.
One of the most common methods of preparing the nano-sized silicon is a method of preparing nano-sized silicon particles, particularly, silicon particles having a diameter of a few tens to a few hundred nanometers, by grinding large silicon particles. However, the above method has limitations in that surface oxidation of silicon may easily occur during the grinding process and the initial efficiency may be reduced due to amorphous SiO2 which is formed on the surface of the silicon as a result of the oxidation. Also, with respect to the rod or fiber-shaped nano-silicon material, since its manufacturing process is complicated and manufacturing costs are high, mass production of the rod or fiber-shaped nano-silicon material may be difficult. Also, with respect to the porous silicon, since pores are formed only in the surface of the powder, a sufficient buffering action is difficult to be obtained during changes in the volume of the active material according to charge and discharge. Thus, lifetime characteristics may degrade.